Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenues	$ 1,760,303	$ 1,840,778	$ 2,002,601
Operating costs and expenses:
Costs of services provided	1,492,317	1,612,877	1,768,162
Selling, general and administrative expense	150,778	150,022	136,603
Other income (expense):
Investment and other income, net	13,352	10,676	5,168
Interest expense	(1,374)	(3,459)	(3,094)
Income before income taxes	129,186	85,096	99,910
Income tax provision	30,504	20,515	16,386
Net income	$ 98,682	$ 64,581	$ 83,524
Per share data:
Basic earnings per common share (in dollars per share)	$ 1.32	$ 0.87	$ 1.13
Diluted earnings per common share (in dollars per share)	$ 1.32	$ 0.87	$ 1.12
Weighted average number of common shares outstanding:
Basic (in shares)	74,696	74,362	74,002
Diluted (in shares)	74,785	74,590	74,612
Comprehensive income:
Net income	$ 98,682	$ 64,581	$ 83,524
Other comprehensive income:
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	2,644	2,761	(679)
Total comprehensive income	$ 101,326	$ 67,342	$ 82,845